Oct. 18, 1999 Bright Rings Found around Sunspots Show Why Spots Are Dark, Cast Shadow on Solar Models
BOULDER--The Chinese noticed dark spots on the sun as early as 25 B.C. and Galileo gazed at them through his telescope in 1611, but they have remained cloaked in mystery over the centuries. Now researchers at the National Center for Atmospheric Research (NCAR) have found bright rings around eight sunspots. The presence of these rings sheds light on why sunspots are dark, and it could spell trouble for conventional models of turbulent diffusion. The research is published this week in the journal Nature. NCAR's primary sponsor is the National Science Foundation.
Scientists have been searching for the rings since they were first predicted 25 years ago. According to lead author Mark Rast, the rings' presence supports the idea that the spots appear dark because their magnetic fields block heat transport. "These findings could change our understanding of how heat is transported under turbulent conditions," he says.
With temperatures of 4000 Kelvin (3700 degrees Celsius), sunspots are both cooler and darker than the surrounding solar disk, or photosphere, which hovers at 6000 K (5700 degrees C) when the sun is quiet. The newly discovered rings are only 1% brighter than the quiet photosphere, and they compensate for only 10% of the sunspots' missing energy. Their contribution to the amount of solar energy reaching the earth is negligible. But their existence could be earthshaking for turbulent diffusion models. Evidence of even faint "bright rings" suggests that convective heat transport around sunspots is structured and vigorous rather than evenly diffuse, as the models indicate.
"The rings suggests that either sunspots are shallow phenomena," says Rast, "or else convective flows around the spots transport heat to the surface more efficiently than the turbulent diffusion models suggest. I believe the latter is the missing component in the models." Such flows may play an important role in sunspot birth and growth.
Scientists have long believed that sunspots are cross-sections of magnetic, rope-like structures whose origins lie deep in the sun's interior. Their missing heat, they say, should appear on the sun's surface as a bright ring around the spot. However, the rings have never been conclusively observed. Current models explain their absence as the result of heat dispersal through homogeneous turbulence in the sun's interior.
In the past, measurement of the rings has been difficult because vertical magnetic flux tubes show up as bright splotches around sunspots, obscuring the faint rings. With the aid of the Precision Solar Photometric Telescope at NCAR's Mauna Loa Solar Observatory in Hawaii, Rast found rings around eight spots in all solar spectrum wavelengths measured. According to Rast, the bigger the spot, the hotter and brighter the ring. He then analyzed data taken with NCAR's Advanced Stokes Polarimeter at the National Solar Observatory in New Mexico, which measures the sun's magnetic field. His analysis showed that only a small fraction of the rings' brightness was due to magnetic intensity of vertical flux tubes. The rest, he believes, is heat suppressed by the magnetic field within the spot, which then emerges as a subtle halo.
The Precision Solar Photometric Telescope was developed at the National Solar Observatory as part of NSF's Radiative Inputs from Sun to Earth (RISE) program, which is dedicated to understanding solar radiative variability as a possible driver of climate change. NCAR is managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.
Visuals: Images are available at ftp://ftp.ucar.edu/communications/Rast1099.
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